Descending a vertical strand, such as a rope or cable, is normally a task that can only be accomplished with some degree of safety by a skilled mountaineer or the like. Thus when a rope is provided as an emergency egress from an upper story of a building or when for some other purpose a person must descend a rope for some considerable distance, it is necessary to provide an apparatus to make such descent not only possible but relatively safe. To this end the apparatus must normally slide along the rope and must normally incorporate some sort of brake to allow a relatively slow rate of descent.
Accordingly Austrian Pat. No. 20,548 shows a device wherein the rope is guided and gripped between a pair of bars whose upper ends are pivoted on a common horizontal element and whose lower ends are connected to a ring. The bars are curved away from each other so that a downward pull on the ring pulls them toward each other so that rough inner surfaces of these arcuate bars will come into contact with the rope or cable and frictionally brake same. This arrangement has the considerable disadvantage that the braking force applied is directly related to the size of the load the devices carry, so that if the load is too light, relative to the coefficient of friction between the bars and the rope, little effective braking will be achieved, yet if the load is too heavy the brake will completely stop on the cable.
Another arrangement is shown in German Pat. No. 233,010. Here the rope or cable is gripped between two rollers carried on one housing part and two other rollers carried on another housing part that is hinged on the one housing part about a horizontal axis parallel to the roller axes. A two-arm lever is pivoted on the bottom of the one housing end and is effective upwardly on the other housing half to press the rollers tightly together and pinch the rope, and carries on its other end an eye for supporting the load. Such an arrangement allows a limited variation of the braking capacity, yet nonetheless is rather dangerous in use, as only limited braking capacity is present, and the operation of the device is a relatively sensitive chore. Consequently the user might descend much too fast or much too slowly.
British Pat. No. 100,942 shows another arrangement wherein a pair of gripping members can be forced inwardly against the rope by means of a pair of levers that are pulled downwardly by the load and hold each other by a spring. This arrangement, once again, has the disadvantage that the braking force is exclusively determined by the size of the load. Furthermore the amount of braking and type of braking can hardly be varied, and the arrangement can only work with strands having diameters lying within a relatively narrow range. This patent also shows a two-handled device having rollers, but nonetheless only operable with strands of a relatively limited size range.
Finally an arrangement can be seen in U.S. Pat. No. 3,739,875 wherein two sets of rollers pinch a rope. The rotation speed of at least one of the sets of rollers is limited by a governor, and a brake that can be controlled by the user is effective on the other rollers. This arrangement is relatively complex. Once again the disadvantage of it is that it can only be used with a rope whose size lies within a very limited range. The rollers are spaced so that a rope that is too thick can not even be fitted between them, whereas a rope that is too thin will not be effectively braked at all.